1. Field of the Invention
This invention relates to monitoring the vibration of turbine blades in an operating turbine and more particularly to detecting and locating resonantly vibrating turbine blades.
2. Description of the Prior Art
In U.S. Pat. No. 4,422,333 by R. L. Leon, the inventor herein, and which issued on Dec. 27, 1983, a method and apparatus was described for detecting and identifying one or more excessively vibrating blades of the rotating portion of a turbomachine. An acoustic sensor was positioned to receive a characteristic Doppler waveform that resulted when a blade or blade group vibrates resonately at an order of running speed. The method described in the '333 patent for constant speed turbines, involved synchronous averaging out the non order-related background noise followed by editing or blanking out the few order related components known to be contaminated with background error such as blade passing frequency and once-per-revolution and twice-per-revolution frequencies. The resulting signal is then displayed to reveal the characteristic Doppler waveform of the blade vibrations. An envelope detection technique was employed to accurately pick out the amplitude peak indicative of the location of the resonant blade. It was discovered that in some steam power turbines the order related background error was not limited to just a few order-related components, but instead was spread broadly across all the order related components. Blanking all of the offending order-related background components would, of course, eliminate entirely the sought after Doppler waveform.